Add `RawFd` to WASI's `std::os::wasi::prelude`.
Add `RawFd` to WASI's `std::os::wasi::prelude`.

Add `RawFd` to WASI's `std::os::wasi::prelude`, making it consistent
with all other platforms which also have `AsRawFd`, `FromRawFd`, and
`IntoRawFd` in their respective preludes.

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust